Normally, a metering servovalve comprises a control chamber having a calibrated inlet hole for pressurized fuel. The control chamber is equipped with an outlet or discharge hole having a calibrated section, which is opened/closed by an axially mobile shutter under the control of an electro-actuator.
Injectors with balanced metering servovalves have already been proposed in which the shutter is subject to substantially null axial pressure forces in the closed position, for which both the spring preload and the force of the actuator can be reduced. In a known balanced metering servovalve, the valve body comprises an axial guide for the armature of the actuator, which is integral with the shutter formed by a sleeve engaging with the axial guide in a fluid-tight manner. The latter is formed by a stem fitted with a discharge channel, comprising an axial segment and at least one calibrated radial segment, which runs to a lateral surface of the stem. As the sleeve must form a seal with the lateral surface of the stem and the shutter must close the discharge channel by engaging with a stop element, it must be machined with extreme precision and be made using a high-quality and very hard material. This material has limited magnetic permeability, and so the electro-actuator must be very powerful. In addition, because the armature is in the form of a notched disc and is integral with the sleeve, the entire sleeve-armature block must be made with this high-quality material for which a lot of waste swarf in this material is produced and machining is very difficult and expensive. Lastly, the armature and sleeve have considerable mass and so the responsiveness of the mobile element is reduced.